1. Field of the Invention
The present invention relates to the field of virtual microscopy and more particularly relates to the assessment of image quality based upon the focus map of a line scan camera prior to and during the capture of imagery data from a specimen on a microscope slide.
2. Related Art
In conventional virtual microscopy systems, image tiling techniques produce individual image tiles that can be significantly out of focus over much of the image. An image tiling system is restricted to a single focus height for each individual snapshot taken by its camera, thus, each of these “fields of view” have areas that are out of focus when the subject specimen being scanned does not have a uniform surface. At the high magnification levels employed in virtual microscopy, specimens with a uniform surface are extremely rare. Moreover, conventional image tiling systems suffer from circular optical distortion that is inherent in the use of an area scan camera with an objective lens. This circular distortion causes the perimeter area of an image tile to be out of focus.
Conventional image tiling solutions are severely handicapped by these limitations, with their only recourse for the inherent circular optical distortion being to discard a significant amount of out of focus image data on the perimeter of each image tile. This results in an increased number of image tiles that must be scanned and a corresponding increase in the time to scan a microscope slide. Furthermore, even when the out of focus perimeter image data is discarded, the remaining image data still suffers from out of focus areas on each image tile due to the lack of a uniform surface and also from circular optical distortion.
Recently, new line scan camera systems have been introduced to the virtual microscopy industry such as the ScanScope® scanner created by Aperio Technologies, Inc. The revolutionary ScanScope® scanner system does not suffer from circular optical distortion due to its use of a line scan camera. Additionally, the line scan camera can adjust its focus for each line of pixels that are captured when scanning a microscope slide and can therefore track a non-uniform surface. Thus, the quality of the resulting image from a line scan camera system is inherently superior due to the sharp focus of each line of pixels captured by the line scan camera.
Although the imagery data captured by a line scan camera is objectively better, there are still focus problems that may arise during image capture. For example, pre-scan focus procedures that identify focus points may focus on non-tissue material, such as dirt, scratches, pen marks, labels, or other debris. This foreign material often resides on the top of the cover slip or the bottom surface of the slide. In these cases the distance between the measured pre-scan focus point and the optimal focus height may be quite large. Accordingly, any tissue that is in the neighborhood of these pre-scan focus points can be dramatically out of focus.
Additionally, when a tissue specimen does not lay flat on the glass (often referred to as having “folded” tissue) the field of view (“FOV”) of the objective lens cannot be brought into focus using a single focus height. In this case, it is important that the pre-scan focus points be focused on the non-folded parts of the tissue, which is increasingly unlikely as the amount of folded tissue increases.
Also, in some cases the tissue specimen does not have sufficient contrast to allow the pre-scan focus procedure to make an accurate measurement. In such a case, the pre-scan focus procedure may focus on other material as previously described. Fortunately, the likelihood of this happening can be greatly reduced by setting a contrast threshold that rejects focus points identified by the pre-scan focus procedure that have insufficient contrast. Finally, a microscope slide may not be held securely during the scanning process. If this occurs, the height of the slide will likely change during the scanning process, after which the focus points identified in the pre-scan focus procedure will no longer be valid.
If any of these conditions occurs, some part of the resulting virtual slide image will be out of focus. However, identifying virtual slide images with out of focus areas requires a manual inspection of each virtual slide image. This is a very costly and time consuming quality control problem. And when out of focus virtual slide images are detected, an operator must use a manual procedure to re-scan the virtual slide image, which is also time consuming. During the manual procedure, the operator will identify known good tissue locations on the slide that are used for focus point placement. After this, the operator will either manually focus the points prior to scanning, or allow the system to proceed automatically using the good focus point locations.
Accordingly, the advancements in the virtual microscopy industry have created a need for improved systems and methods for assessing virtual slide image quality that overcome the costly and time consuming manual inspection process and capitalizes on the focusing capabilities and speed of the revolutionary line scan camera systems.